Perforating guns are used to create openings or perforations in pipe or tubular goods. In an oil or gas well, a perforating gun is lowered into the well inside of the pipe, down to the level of the oil or gas bearing formation. The perforating gun is then fired and the pipe is perforated to allow fluids from the formation to enter the pipe. Perforating guns use shaped charges or some other devices to create the actual openings in the pipe.
When a pipe is perforated, typically a long string of perforating guns is used. It is desirable to have control and fire off specific segments of the perforating gun string, rather than fire the entire perforating gun string at once.
The prior art accomplishes this task of selective activation by using a single wire system. The perforating gun string is made up of perforating gun subs, which contain the shaped charges that perform the actual perforating, alternated with switch subs. The switch sub contains an electrical switch. There is a switch for each perforating gun sub. The perforating gun string is fired from the bottommost perforating gun sub up towards the surface. The switches, typically located adjacent to the bottom portion of the respective perforating gun sub, provides electrical contact therethrough to the next lowermost perforating gun sub, while preventing electrical contact with the circuit inside of the associated and respective perforating gun sub. Thus, electrical current can be passed through a particular sub to the lowermost perforating gun sub. When needed, the switch makes electrical contact between the power supply on the surface with its respective perforating gun sub.
When a perforating gun sub is detonated, it creates a blast of pressure and heat that is used to close the adjacent upper switch in the adjacent upper sub. Thus, the next upper perforating gun sub is activated and ready for use.
The switches are an important component of the perforating gun string. If a switch fails, the operation of the entire string can be jeopardized. Because the switch is activated or closed by a blast from a perforating gun, prior art switches tend to have reliability problems. A perforating gun may generate a blast that overpressures and overwhelms the switch.